Optical communications systems using optical fibers to convey information between a light source and a light detector are presently of significant and growing commercial interest. The light source used in such systems is commonly a semiconductor laser. Characteristics of the semiconductor laser, such as emission wavelength, that are important for system operation are functions of the laser temperature. However, the current flowing through the laser to produce radiation also produces heating of the laser which is frequently because it, for example, shifts the emission wavelength. Accordingly, techniques have been developed to cool the lasers so that they may be operated at a constant temperature. Perhaps the most prevalently used technique at the present time to maintain the laser temperature is thermoelectric cooling. This technique is well known to those skilled in the art and need not be further described.
Initial optical communications systems used repeaters to span long distances. The repeaters detected and then regenerated optical signal. However, complex electronics are required for detection and regeneration. A conceptually simpler and more recently developed technique uses optical amplifiers to span long distances. The most commonly used optical amplifiers have a segment of a rare earth doped length of optical fiber which is pumped by a pump laser. The doped segment of optical fiber amplifies the incoming optical signal and emits an amplified optical signal. An exemplary dopant is erbium and erbium doped fiber amplifiers are often referred to by the acronym EDFA.
The frequency of the pump laser is also an important system parameter and the pump lasers used in the fiber amplifiers are often cooled with thermoelectric coolers. The fiber amplifiers may be remotely situated and reducing power consumption is desirable. Of course, the thermoelectric cooler also consumes power. Many fiber amplifier packages or modules have dual pump lasers. These modules must have both lasers cooled and power must be supplied to the thermoelectric cooler. The degree of precision required for temperature control mandates precise control of the currents through the each laser. This control has been obtained by using parallel connected thermoelectric coolers. This configuration permits easy control of the thermoelectric coolers but has high power consumption which is undesirable because it requires use of big power supplies, a bulky heat sink, or specially manufactured printed circuit boards to remove the heat from the components of the module.
It is also recognized that control of the current through the laser is critical as it is for the cooling apparatus.